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Displacement vs. weight.

I have a general question based on nothing more than logic. It involves the actual weight of a boat vs. its displacement. My logic tells me that the displacement is the amount of water weight that is "displaced" by the boat in the water. As a hull weighs more it displaces more to the point where if the hull weight = the displaced weight it would sit at the gunwales in water. So again, logically thinking, a baot has to weigh less than it's displacement to float. My question is...is this a true statement?

Not taking into consideration movement, just statically flaoting.

Thanks,

DG

"The desire to build a house is the tired wish of a man content thenceforward with a single anchorage. The desire to build a boat is the desire of youth, unwilling yet to accept the idea of a final resting place." -Arthur Ransome

Not quite. The weight of the water displaced by the boat must be equal to the weight of the boat. If a boat weighs 2000 lbs. it will displace 2000 lbs. of water (or about 31 cubic feet of seawater). If this boat cannot push aside (displace) 31 cubic feet of seawater without putting the gunwales under then it will sink.

Displacement and weight of a boat are equal. The volume of water equivalent to the displacement has to be less than the volume of the hull for the boat to float.

Precisely! However many designers muddy the definition of displacement by quoting not the boat's bare weight but it's weight partially loaded or to it's designed water water line. Ted Brewer puts it this way:

DISPLACEMENT: If you weigh the boat on a scale, that is her actual (not advertised) displacement and it is the weight of sea water that she will displace when she is afloat. Most designers figure displacement when half loaded (the boat, not the designer) with stores, liquids and crew.

Volumn of the boat vs volumn of water displaced. That is what my logic was missing.
My thoughts were around towing. and I was hoping actual weight would be less. So a boat that is 28'x8' isn't to large to tow, but if it displaces (weighs) 10K lbs then it is to big for my truck.

"The desire to build a house is the tired wish of a man content thenceforward with a single anchorage. The desire to build a boat is the desire of youth, unwilling yet to accept the idea of a final resting place." -Arthur Ransome

I've got to jump in here to split a few hairs. You are talking about very specific terms here, so they need very specific definitions.

The displacement of a boat or ship is the mass of the water that the hull displaces at a given loading condition. This is variable depending on the amount of fuel, stores, cargo, persons, etc., on board.

The weight, or more accurately the lightship displacement of a vessel is the mass of only the structure and outfit of the boat or ship, without any fuel, cargo, etc.

Commercially, both the lightship and maximum displacement figures are usually available so that the charterer or owner knows pretty accurately what the cargo-carrying capacity of the vessel is. Pleasure vessels have no such compunction, so the buyer is at a loss to determine how much he can safely put on board and maintain expected performance and safety parameters.

This can get quite confusing if not made clear when stating type of displacements. For instance, quite often sailboats are advertised as "weighing" a certain amount, but it is not made clear whether this is lightship, half-capacity, at design waterline, full load, or whatever. Speedboats are often advertised as a certain weight to enhance the idea that you can trailer them behind your Yugo, failing to note that the stated weight is without 500 pounds of fuel, water, lifejackets, electronics, etc. They often fail to mention, as well, that the performance figures given are derived from testing with less than a quarter of a tank of fuel and no supplies aboard, and that the speed runs are the last thing done in the testing day so that the fuel tanks are almost dry to reduce displacement.

Towing weight will be less. Designed displacement includes some or all the people & stuff carried in the boat. A lot of this wouldn't be in the boat when towing. Fuel can make a big difference too. However, don't forget the trailer weight.

1) The volume of water displaced by a floating object will differ depending on the density of the water - which means that your boat will float higher in Utah's Great Salt Lake, for example, than in a nice freshwater lake. Probably an expensive way to expose your waterline for painting, though.

2) If the object is completely immersed (sunk, for example) it will displace an amount equal to the volume of material, and not the weight. Not, of course, that any of our boats will ever sink to the bottom....

Sometimes you've gotta leave the kibble out where the slow dogs can get some....
... Roy Blount, Jr.

Displacement can be expressed in pounds, long tons or cubic feet; one ton = 2240 pounds = 35 cubic feet of sea water, at 64 pounds per cubic foot. Fresh water weighs only 62.4 pounds per c.f. so a boat taken from sea water to fresh water will sink into the water and increase her draft slightly. For example, a boat weighing 7500 pounds will displace 117.19 cu. ft. of sea water or 120.19 cu. ft. of fresh water. The difference is 3 cu. ft. so, if her waterline area is 150 sq. ft., she will sink 3/150 of a foot (about 1/4") when she is moved from salt to fresh water. It is truly insignificant for most sailors, unless you are skippering a 90,000 ton tanker.

When a designer, builder or salesman states the "displacement", it doesn't tell you much about the boat. If everyone would use a convention like MMD offers, it would make things a lot easier in comparing boats. The bare weight of the boat without people, gear or stores and the loaded displacement tells you how much load the boat will allow to set it on its designed operating lines without overloading. The added weight to cause the boat to sink one inch at the waterline is a common and very useful tool also.

And besides that . . .

How many designers tell the loading at the specified displacement? James Wharram does, for one. He (they) tells you displacement empty and he gives the maximum payload weight. Add them together and you get total displacement at maximum loading. But he doesn't tell you at what displacement the LWL is drawn, so you don't know maximum draft or minumum clearance between sea and platform. And it is handy to know that his displacement/weight figures are in short tons, rather than long tons.

Anybody know of any other designers that give you that much without buying the plans?

metrics is good in this case. Without getting into the fine tuning of salt and fresh water, a cubic metre of water weighs 1 tonne or 1000 kgs.
That would be why that 20 litre bucket has 20 kg of water in it and two can pull your arms off after about 100 metres.